Hey I am trying to make 10 threads in a synchronized manner and I came up with the code below however I am not able to understand a part of it as mentioned below. I am still really new to java, I tried looking up synchronized threading from
Here but still I am clueless.
class question3 {
public static void main(String arg[]) throws Exception {
for (int i = 0; i < 11; i++) {
data di = new data();
System.out.println(di.count);
}
}
}
class item {
static int count = 0;
}
class data extends item implements Runnable {
item d = this;
Thread t;
data() {
t = new Thread(this);
t.start();
}
public void run() {
d = syn.increment(d);
}
}
class syn {
synchronized static item increment(item i) {
i.count++;
return (i);
}
}
I am not sure what this part of code does?
public void run() {
d = syn.increment(d);
}
}
class syn {
synchronized static item increment(item i) {
i.count++;
return (i);
}
}
the run function is used when starting the thread, this is a must function you need to override when implement Runnable. When calling Thread.start(), the run function will be called.
The class syn contains a synchronized method, it is simply mean that only one thread can access it each time, thus make the incerment function thread safe.
object d has a static variable count meaning all instances of item class (and data ) share the same count, so all threads increment the same variable
the line d = syn.increment(d); is basically count++ but in a thread safe way
I don't know if the following thing is possible. I would like a Runnable's run() method to contain the Runnable itself, i.e.
reconnects = 3;
Runnable executeAfter = () -> {
if ( --reconnects < 0 ) {
println("%nStop using port %d.", this.port);
//...
} else { // try to reconnect
println("%nReconnecting...");
cmdRun = new CmdRun(command, executeAfter);
(new Thread(cmdRun)).start();
//...
}
};
Is something like this even possible? If so, how? (CmdRun's constructor is CmdRun(String command, Runnable executeAfter))
Is lambda a must here? If not, switching to older equivalent syntax should be simple:
An example:
public class TestLambda {
static int count = 0;
public static void main(String[] args) {
// lambda not going to work
//Runnable foo = () -> { if (count < 5) { call(foo); } };
// nor
//Runnable foo = () -> { if (count < 5) { call(this); } };
// using old way of anonymous inner class will work
Runnable foo = new Runnable() {
#Override public void run() {
if (count < 5) {
call(this);
}
}
};
foo.run();
}
public static void call(Runnable action) {
count++;
System.out.println("in call " + count);
action.run();
}
}
The easiest way is probably to put the content of your lambda in a method and use a method reference to define your Runnable.
The Runnable's run() can not contain a self reference as its illegal.
Im not exactly sure what you are trying to achieve but something like this should work :
class CmdRun implements Runnable {
final Object command;
final Runnable runnable;
final Runnable executeAfter = () -> {
if ( --reconnects < 0 ) {
System.out.println("%nStop using port %d." + port);
//...
} else { // try to reconnect
System.out.println("%nReconnecting...");
CmdRun cmdRun = new CmdRun(command);
(new Thread(cmdRun)).start();
//...
}
};
public CmdRun(Object command) {
this.command = command;
this.runnable = executeAfter;
}
#Override
public void run() {
runnable.run();
}
}
Short answer: No.
Long answer:
Your code will give you a syntax error. Why? The executeAfter used inside the lambda is not initialized; it is only initialized after the full body of the lambda definition.
For example, consider the below example.
int i;
sum(i, 5); // Syntax error!! Variable i is not initialized...
Your case is similar. Inside the lambda, executeAfter is not initialized. As stated above, it is only initialized after the full body of the lambda's definition.
One additional thing to node is that the variable reconnects must be a final in order to be used inside the lambda. If it is a final variable, then you cannot use the -- operator on it inside your if condition.
Actually if you don't mind to introduce a new interface (or if you require such functionality more often), you could use the following:
#FunctionalInterface
interface RecursiveRunnable extends Runnable {
default void run() {
run(this);
}
public void run(RecursiveRunnable runnable);
}
This will now allow you to recursively call the runnable, e.g.:
int maxTries = 3;
AtomicInteger counter = new AtomicInteger();
RecursiveRunnable foo = runnable -> {
if (counter.getAndIncrement() < maxTries) {
println("Reconnecting... %n");
runnable.run(); // same as: runnable.run(runnable)
} else {
println("Stop using port %d%n", port);
}
};
If I do the following, I will be able to create an object as a thread and run it.
class ThreadTest
{
public static voic main(String[] args)
{
HelloThread hello = new HelloThread();
Thread t = new Thread(hello);
t.start();
}
}
class HelloThread extends Thread
{
public void run()
{
System.out.println(" Hello ");
}
}
Now, if my HelloThread class has a another method call runThisPlease(), how are we supposed to run it with a thread?
Example:
class HelloThread extends Thread
{
public void run()
{
System.out.println(" Hello ");
}
public void runThisPlease(String input)
{
System.out.println (" Using thread on another class method: " + input );
}
}
Que: When I try Thread t = new Thread(hello.runThisPlease());, it doesn't work. So how can we call the method runThisPlease() using a thread?
Edit: Argument needed in method for runThisPlease();
In java 8 you can use
Thread t = new Thread(hello::runThisPlease);
hello::runThisPlease will be converted to a Runnable with a run method that calls hello.runThisPlease();.
If your want to call a method, that needs parameters, e.g. System.out.println, you can of course use a normal lambda expression too:
final String parameter = "hello world";
Thread t = new Thread(() -> System.out.println(parameter));
If you use a java version < 8, you can of course replace the method reference / lambda expression with anonymus inner classes that extend Runnable (which is what a java8 compiler does, AFAIK), see other answers.
However you can also use a anonymus inner class that extends Thread:
final HelloThread hello = //...
Thread t = new Thread() {
#Override
public void run() {
hello.runThisPlease();
}
};
Simply calling the runThisPlease() from within the run() method will make it part of a new thread.
Try this:
class HelloThread extends Thread
{
public void run()
{
System.out.println(" Hello .. Invoking runThisPlease()");
runThisPlease();
}
public void runThisPlease()
{
System.out.println (" Using thread on another class method ");
}
}
Things are maybe more clear if you use the Runnable interface:
public class HelloThread implements Runnable
{
#Override
public void run() {
// executed when the thread is started
runThisPlease();
}
public void runThisPlease() {...}
}
To launch this call:
Thread t=new Thread(new HelloThread());
t.start();
The Thread class can not see your extra method because it is not part of the Runnable interface.
As a convenience Thread implements Runnable but I don't think it helps in clarity :(
You have to only call this method inside run() method.
public void run(){
System.out.println(" Hello ");
runThisPlease();
}
If you want to pass some argument then you can use below code
String str = "Welcome";
Thread t = new Thread(new Runnable() {
public void run() {
System.out.println(str);
}});
I was reading the book "Head First Java" and at some point it mentioned that an inner class instance must be tied to an outer class instance, which I was already aware of, but with an exception:
A very special case—an inner class defined within a static method. But
you might go your entire Java life without ever encountering one of
these.
I'm pretty sure that last statement is indeed true, but if the compiler allows it to happen it means that it exists for a reason, otherwise it would be illegal Java. Can someone show me an example of where this would be useful?
It may be special, it may not be.
You're looking at a local class available within a method:
class Foo {
static void bar(){
class MyRunnable implements Runnable {
public void run() {
System.out.println("No longer anonymous!");
}
};
Thread baz = new Thread(new MyRunnable());
}
}
I've seen uses of inner classes that are anonymous like:
class Foo {
static void bar(){
Thread baz=new Thread(new Runnable(){
public void run(){
System.out.println("quux");
}
}
}
}
This is technically an inner class(though anonymous) and defined in a static method. I personally would create a static nested class that implements Runnable and do:
baz = new Thread(new MyRunnable());
where MyRunnable is defined as:
class Foo {
static void bar(){
// SNIP
}
static class MyRunnable implements Runnable {
public void run() {
System.out.println("No longer anonymous!");
}
}
}
Some people take the view that any method that can be static should be static. To such a person, the inner-beauty of the class would not be terribly relevant.
Here's a fabricated example of an inner class within a static method. It can be claimed that
It does not need to be declared outside the static method as it is not required elsewhere
It should be a named class (i.e. not anonymous) as it is being used multiple times
class Race {
public static void main(String[] args) throws Exception{
class Runner implements Runnable {
final String name;
long time = -1;
Runner(String name) { this.name = name; }
public void run() {
try {
long start = System.currentTimeMillis();
time = -2;
System.out.printf("Start %s\n", name);
for (int i = 0; i < 10; i++) {
Thread.sleep(1000);
}
System.out.printf("End %s\n", name);
this.time = System.currentTimeMillis() - start;
} catch (InterruptedException e) {
time = -3;
}
}
long time() { return time; }
}
Runner r1 = new Runner("One");
Runner r2 = new Runner("Two");
Thread one = new Thread(r1);
Thread two = new Thread(r2);
one.start();
two.start();
one.join();
two.join();
System.out.printf("One: %s, Two: %s\n", r1.time(), r2.time());
System.out.printf("%s wins\n", r1.time() < r2.time() ? "one" : "two");
}
}
I don't know the full context, but closures (i.e. Guava's Function implementation) and implementations defined in an utility class could be an example.
However, after searching for a while, I haven't found anonymous closure examples in Guava itself.
Can anyone suggest to me how I can pass a parameter to a thread?
Also, how does it work for anonymous classes?
You need to pass the parameter in the constructor to the Runnable object:
public class MyRunnable implements Runnable {
public MyRunnable(Object parameter) {
// store parameter for later user
}
public void run() {
}
}
and invoke it thus:
Runnable r = new MyRunnable(param_value);
new Thread(r).start();
For Anonymous classes:
In response to question edits here is how it works for Anonymous classes
final X parameter = ...; // the final is important
Thread t = new Thread(new Runnable() {
p = parameter;
public void run() {
...
};
t.start();
Named classes:
You have a class that extends Thread (or implements Runnable) and a constructor with the parameters you'd like to pass. Then, when you create the new thread, you have to pass in the arguments, and then start the thread, something like this:
Thread t = new MyThread(args...);
t.start();
Runnable is a much better solution than Thread BTW. So I'd prefer:
public class MyRunnable implements Runnable {
private X parameter;
public MyRunnable(X parameter) {
this.parameter = parameter;
}
public void run() {
}
}
Thread t = new Thread(new MyRunnable(parameter));
t.start();
This answer is basically the same as this similar question: How to pass parameters to a Thread object
via constructor of a Runnable or Thread class
class MyThread extends Thread {
private String to;
public MyThread(String to) {
this.to = to;
}
#Override
public void run() {
System.out.println("hello " + to);
}
}
public static void main(String[] args) {
new MyThread("world!").start();
}
This answer comes very late, but maybe someone will find it useful. It is about how to pass a parameter(s) to a Runnable without even declaring named class (handy for inliners):
String someValue = "Just a demo, really...";
new Thread(new Runnable() {
private String myParam;
public Runnable init(String myParam) {
this.myParam = myParam;
return this;
}
#Override
public void run() {
System.out.println("This is called from another thread.");
System.out.println(this.myParam);
}
}.init(someValue)).start();
Of course you can postpone execution of start to some more convenient or appropriate moment. And it is up to you what will be the signature of init method (so it may take more and/or different arguments) and of course even its name, but basically you get an idea.
In fact there is also another way of passing a parameter to an anonymous class, with the use of the initializer blocks. Consider this:
String someValue = "Another demo, no serious thing...";
int anotherValue = 42;
new Thread(new Runnable() {
private String myParam;
private int myOtherParam;
// instance initializer
{
this.myParam = someValue;
this.myOtherParam = anotherValue;
}
#Override
public void run() {
System.out.println("This comes from another thread.");
System.out.println(this.myParam + ", " + this.myOtherParam);
}
}).start();
So all happens inside of the initializer block.
When you create a thread, you need an instance of Runnable. The easiest way to pass in a parameter would be to pass it in as an argument to the constructor:
public class MyRunnable implements Runnable {
private volatile String myParam;
public MyRunnable(String myParam){
this.myParam = myParam;
...
}
public void run(){
// do something with myParam here
...
}
}
MyRunnable myRunnable = new myRunnable("Hello World");
new Thread(myRunnable).start();
If you then want to change the parameter while the thread is running, you can simply add a setter method to your runnable class:
public void setMyParam(String value){
this.myParam = value;
}
Once you have this, you can change the value of the parameter by calling like this:
myRunnable.setMyParam("Goodbye World");
Of course, if you want to trigger an action when the parameter is changed, you will have to use locks, which makes things considerably more complex.
I know that I'm a few years late, but I came across this issue and took an unorthodox approach. I wanted to do it without making a new class, so this is what I came up with:
int x = 0;
new Thread((new Runnable() {
int x;
public void run() {
// stuff with x and whatever else you want
}
public Runnable pass(int x) {
this.x = x;
return this;
}
}).pass(x)).start();
You can either extend the Thread class or the Runnable class and provide parameters as you want. There are simple examples in the docs. I'll port them here:
class PrimeThread extends Thread {
long minPrime;
PrimeThread(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
. . .
}
}
PrimeThread p = new PrimeThread(143);
p.start();
class PrimeRun implements Runnable {
long minPrime;
PrimeRun(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
. . .
}
}
PrimeRun p = new PrimeRun(143);
new Thread(p).start();
To create a thread you normally create your own implementation of Runnable. Pass the parameters to the thread in the constructor of this class.
class MyThread implements Runnable{
private int a;
private String b;
private double c;
public MyThread(int a, String b, double c){
this.a = a;
this.b = b;
this.c = c;
}
public void run(){
doSomething(a, b, c);
}
}
Either write a class that implements Runnable, and pass whatever you need in a suitably defined constructor, or write a class that extends Thread with a suitably defined constructor that calls super() with appropriate parameters.
In Java 8 you can use lambda expressions with the Concurrency API & the ExecutorService as a higher level replacement for working with threads directly:
newCachedThreadPool() Creates a thread pool that creates new threads
as needed, but will reuse previously constructed threads when they are
available. These pools will typically improve the performance of programs that execute many short-lived asynchronous tasks.
private static final ExecutorService executor = Executors.newCachedThreadPool();
executor.submit(() -> {
myFunction(myParam1, myParam2);
});
See also executors javadocs.
As of Java 8, you can use a lambda to capture parameters that are effectively final. For example:
final String param1 = "First param";
final int param2 = 2;
new Thread(() -> {
// Do whatever you want here: param1 and param2 are in-scope!
System.out.println(param1);
System.out.println(param2);
}).start();
Parameter passing via the start() and run() methods:
// Tester
public static void main(String... args) throws Exception {
ThreadType2 t = new ThreadType2(new RunnableType2(){
public void run(Object object) {
System.out.println("Parameter="+object);
}});
t.start("the parameter");
}
// New class 1 of 2
public class ThreadType2 {
final private Thread thread;
private Object objectIn = null;
ThreadType2(final RunnableType2 runnableType2) {
thread = new Thread(new Runnable() {
public void run() {
runnableType2.run(objectIn);
}});
}
public void start(final Object object) {
this.objectIn = object;
thread.start();
}
// If you want to do things like setDaemon(true);
public Thread getThread() {
return thread;
}
}
// New class 2 of 2
public interface RunnableType2 {
public void run(Object object);
}
You can derive a class from Runnable, and during the construction (say) pass the parameter in.
Then launch it using Thread.start(Runnable r);
If you mean whilst the thread is running, then simply hold a reference to your derived object in the calling thread, and call the appropriate setter methods (synchronising where appropriate)
There is a simple way of passing parameters into runnables.
Code:
public void Function(final type variable) {
Runnable runnable = new Runnable() {
public void run() {
//Code adding here...
}
};
new Thread(runnable).start();
}
No you can't pass parameters to the run() method. The signature tells you that (it has no parameters). Probably the easiest way to do this would be to use a purpose-built object that takes a parameter in the constructor and stores it in a final variable:
public class WorkingTask implements Runnable
{
private final Object toWorkWith;
public WorkingTask(Object workOnMe)
{
toWorkWith = workOnMe;
}
public void run()
{
//do work
}
}
//...
Thread t = new Thread(new WorkingTask(theData));
t.start();
Once you do that - you have to be careful of the data integrity of the object you pass into the 'WorkingTask'. The data will now exist in two different threads so you have to make sure it is Thread Safe.
One further option; this approach lets you use the Runnable item like an asynchronous function call. If your task does not need to return a result, e.g. it just performs some action you don't need to worry about how you pass back an "outcome".
This pattern lets you reuse an item, where you need some kind of internal state. When not passing parameter(s) in the constructor care is needed to mediate the programs access to parameters. You may need more checks if your use-case involves different callers, etc.
public class MyRunnable implements Runnable
{
private final Boolean PARAMETER_LOCK = false;
private X parameter;
public MyRunnable(X parameter) {
this.parameter = parameter;
}
public void setParameter( final X newParameter ){
boolean done = false;
synchronize( PARAMETER_LOCK )
{
if( null == parameter )
{
parameter = newParameter;
done = true;
}
}
if( ! done )
{
throw new RuntimeException("MyRunnable - Parameter not cleared." );
}
}
public void clearParameter(){
synchronize( PARAMETER_LOCK )
{
parameter = null;
}
}
public void run() {
X localParameter;
synchronize( PARAMETER_LOCK )
{
localParameter = parameter;
}
if( null != localParameter )
{
clearParameter(); //-- could clear now, or later, or not at all ...
doSomeStuff( localParameter );
}
}
}
Thread t = new Thread(new MyRunnable(parameter));
t.start();
If you need a result of processing, you will also need to coordinate completion of MyRunnable when the sub-task finishes. You could pass a call back or just wait on the Thread 't', etc.
Specially for Android
For callback purposes I usually implement my own generic Runnable with input parameter(s):
public interface Runnable<TResult> {
void run(TResult result);
}
Usage is simple:
myManager.doCallbackOperation(new Runnable<MyResult>() {
#Override
public void run(MyResult result) {
// do something with the result
}
});
In manager:
public void doCallbackOperation(Runnable<MyResult> runnable) {
new AsyncTask<Void, Void, MyResult>() {
#Override
protected MyResult doInBackground(Void... params) {
// do background operation
return new MyResult(); // return resulting object
}
#Override
protected void onPostExecute(MyResult result) {
// execute runnable passing the result when operation has finished
runnable.run(result);
}
}.execute();
}
Create a local variable in your class that extends Thread or implements Runnable.
public class Extractor extends Thread {
public String webpage = "";
public Extractor(String w){
webpage = w;
}
public void setWebpage(String l){
webpage = l;
}
#Override
public void run() {// l is link
System.out.println(webpage);
}
public String toString(){
return "Page: "+webpage;
}}
This way, you can pass a variable when you run it.
Extractor e = new Extractor("www.google.com");
e.start();
The output:
"www.google.com"
First I want to point out that other answers are true.
However, using the parameter in the constructor may not be the best idea for all of you.
In many scenarios you will want to use "Anonymous Inner Class", and override the run() method, because defining specific class for every use is painful.
(new MyRunnable(){...})
And at the time you create that Runnable, the parameter may not be available to you to pass it in the constructor. If for example, you pass this object to a method, that will perform some work in separate thread and then call your runnable, applying the result from that work to it.
In that case, using a method like this one:
public MyRunnable withParameter(Object parameter), may turn out to be far more useful choice.
I do not claim that this is the best solution to the problem, but it will get the job done.